Advances in polymerization and catalysis have resulted in the ability to produce many new polymers having improved physical and chemical properties useful in a wide variety of superior products and applications. With the development of new catalysts, the choice of polymerization (solution, slurry, high pressure or gas phase) for producing a particular polymer have been greatly expanded. Also, advances in polymerization technology have provided more efficient, highly productive and economically enhanced processes.
As with any new technology field, particularly in the polyolefins industry, a small savings in cost often determines whether a commercial endeavor is even feasible. The industry has been extremely focused on developing new and improved catalyst systems. Some have focused on designing the catalyst systems to produce new polymers, others on improved operability, and many more on improving catalyst productivity. The productivity of a catalyst, that is, the amount of polymer produced per gram of the catalyst, usually is the key economic factor that can make or break a new commercial development in the polyolefin industry.
Ziegler-Natta catalyst systems are utilized extensively in commercial processes that produce high density and low-density polyethylenes in a variety of molecular weights. Production rates in certain gas phase reactors may be limited in their ability to discharge from the reactor the polymer particles that are produced during the reaction. In certain of such cases, an increase in the bulk density of the polymer particles may increase the production rate of the reactor. Generally, Ziegler-Natta catalysts that have increasing activity and productivity, and that are used in gas phase operations. may tend to produce polymer products that have decreasing bulk density. If a reactor is limited in its ability to discharge the polymer product, the use of a high activity catalyst may result in a decrease in the bulk density of the polymer product.
Background references include U.S. Pat. No. 4,405,495 and EP 0 043 220 A.
Considering the discussion above, a need exists for higher productivity catalyst systems capable of providing the efficiencies necessary for implementing commercial polyolefin processes. Thus, it would be highly advantageous to have a polymerization process and catalyst system capable of producing polyolefins with improved catalyst productivities and reactor performance.
While the present invention is susceptible to various modifications and alternative forms, specific exemplary embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.